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          <h2 class="post-title" itemprop="name headline">Java 多态性</h2>
        

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        <p>Java多态性</p>
<p>我们已经学习完了关于继承的所有概念，但是并不意味着只要使用继承就可以实现代码的全部重用，而在继承之后又有了一个重要的核心概念：多态性；</p>
<p>在Java里面对于多态的核心表现主要有以下两点：</p>
<p>1.方法的多态性</p>
<pre><code>- 方法的重载：同一个方法名称可以根据参数的类型及个数的不同调用不同的方法体；
- 方法的覆写：同一个父类的方法，可能根据实例化子类的不同也有不同的实现；
</code></pre><p>2.对象的多态性（前提：方法覆写）</p>
<pre><code>- 对象的向上转型（自动的，90%）：父类 父类对象 = 子类实例；
- 对象的向下转型（强制的，1%）：子类 子类对象 = （子类）父类实例；
- 9%不进行转型，例如String
</code></pre><p>范例：回顾一个简单程序</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【A】"</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">B</span> <span class="keyword">extends</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【B】"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demp</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">    B b = <span class="keyword">new</span> B();<span class="comment">//实例化子类对象</span></span><br><span class="line">    b.print();<span class="comment">//调用被覆写过的方法</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>于是我们可以将以上的代码进行进一步的变化，变为向上转型：</p>
<p>范例：实现向上转型</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【A】"</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">B</span> <span class="keyword">extends</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【B】"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demp</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">    A a = <span class="keyword">new</span> B();<span class="comment">//向上转型</span></span><br><span class="line">    a.print();</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>不管是否发生了向上转型，其核心的本质还在于：你是用的是哪一个子类（new 在哪里），而后你调用的方法是否被子类所覆写了。</p>
<p>向下转型指的是将父类对象变为子类对象，但是在这之前必须先明白一个核心概念，为什么我们需要向下转型？当你使用到子类扩充操作的时候就要采用向下转型。</p>
<p>范例：观察向下转型</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【A】"</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">B</span> <span class="keyword">extends</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【B】"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">fun</span><span class="params">()</span></span>&#123;   <span class="comment">//这个方法只有子类有</span></span><br><span class="line">    System.out.print(<span class="string">"fun()"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demp</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">    A a = <span class="keyword">new</span> B();<span class="comment">//向上转型</span></span><br><span class="line">    a.print();</span><br><span class="line">    <span class="comment">//这个时候父类能够调用的方法只有本类定义好的方法print()，所以并没有B类中的fun()方法，那么只能够向下转型</span></span><br><span class="line">    B b = (B) a;<span class="comment">//向下转型</span></span><br><span class="line">    b.fun();</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>【此概念在开发中一般用不到】并不是所有的父类对象都可以向下转型，如果要想向下转型，一定要首先发生向上转型，否则在转型时会发生：ClassCastException。</p>
<p>范例：错误的转型</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【A】"</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">B</span> <span class="keyword">extends</span> <span class="title">A</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">print</span><span class="params">()</span></span>&#123;</span><br><span class="line">    System.out.print(<span class="string">"【B】"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">fun</span><span class="params">()</span></span>&#123;   <span class="comment">//这个方法只有子类有</span></span><br><span class="line">    System.out.print(<span class="string">"fun()"</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demp</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">    A a = <span class="keyword">new</span> A();<span class="comment">//实例化父类对象</span></span><br><span class="line">    B b = (B) a;  <span class="comment">//强制转换</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>以上程序就会产生ClassCastException的错误。</p>
</blockquote>
<p>但是现在就有一个问题出现了，如果向下转型可能存在有隐患，那么如何转型才靠谱呢？最好的做法事先进行判断，而后在进行转型，那么就可以使用instanceof关键字来实现了，此关键字的使用语法如下：</p>
<ul>
<li><code>子类对象 instanceof 类</code>返回的是boolean数据</li>
</ul>
<p>范例：观察instanceof关键字的使用</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demo</span></span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">    A a = <span class="keyword">new</span> B();</span><br><span class="line">    System.out.println(a <span class="keyword">instanceof</span> A);<span class="comment">//true</span></span><br><span class="line">    System.out.println(a <span class="keyword">instanceof</span> B);<span class="comment">//true</span></span><br><span class="line">    <span class="keyword">if</span>(a <span class="keyword">instanceof</span> B)&#123;   <span class="comment">//就避免出现ClassCastException的错误</span></span><br><span class="line">      B b = (B) a;  </span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>虽然清楚了这一系列的操作关系，可是还必须思考这种转换有什么意义？</p>
<p>范例：要求定义一个方法，要求这个方法可以接受我们Person类的的所有子类实例,并调用我们Person类的方法</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> 多态性;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Person</span></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">say</span><span class="params">()</span></span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"我是一个人！"</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Worker</span> <span class="keyword">extends</span> <span class="title">Person</span></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">say</span><span class="params">()</span></span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"我是一个工人！"</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Student</span> <span class="keyword">extends</span> <span class="title">Person</span></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">say</span><span class="params">()</span></span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"我是一个学生！"</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Demo</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span></span>&#123;</span><br><span class="line">        Student stu = <span class="keyword">new</span> Student();</span><br><span class="line">        Worker wor = <span class="keyword">new</span> Worker();</span><br><span class="line">        in(stu);</span><br><span class="line">        in(wor);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">in</span><span class="params">(Person per)</span></span>&#123;</span><br><span class="line">        per.say(); <span class="comment">//所有的人的子类都可以进入这个方法。</span></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h1 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h1><ol>
<li>对象多态性的实现的核心在于方法的覆写；</li>
<li>通过对象的向上转型可以实现接受参数的统一，而向下转型可以实现子类扩充方法的调用（一般不会操作向下转型）；</li>
<li>两个没有关系的对象是不能够转型的，一定会发生ClassCastException，所以向下转型是存在有安全隐患的。</li>
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